Fiber-optic current sensors are based on the magneto-optic circular birefringence in an optical fiber that is coiled around the current conductor. This birefringence becomes manifest as a phase-shift between left and right circularly polarized light states which is accumulated along the sensing fiber. The total phase shift is proportional to the line integral of the magnetic field, which, in the case of a closed (or looped) sensing fiber coil where the two ends of the sensing fiber are in close spatial proximity, is simply proportional to the enclosed current I times number of fiber windings N.
More precisely, the magneto-optic phase shift between left and right circularly polarized light states amounts to 4VNI (fiber coil operated in a reflection-type sensor) or 2VNI (fiber coil operated in a transmission-type sensor), where V is the Verdet constant of the silica fiber. For example, V≈1.0×10−6 A−1 at 1310 nm in fused silica fiber.
Different detection schemes and sensors exist to interrogate this phase-shift: Simple schemes work within a range <±π/2 [1, 2, 19], whereas more sophisticated closed-loop schemes often cover a range from −π to +π [3]. Accordingly, the number of sensor fiber windings around the current conductor determines the measurement range of the sensor in terms of current since a higher number of windings increases the magneto-optic phase shift and thus the signal-to-noise ratio, but reduces the maximum current that can be measured for the respective fixed range of detectable phase shifts. For instance, in the case of one fiber winding, the maximum detectable current at 1310 nm corresponds, for a maximum detectable phase shift of π/2 (or π), to roughly 390 kA (or 780 kA) for a current sensor operated in reflection. This current range can be further affected by non-linear sensor characteristics at high magneto-optic phase shifts.
Measurement of high electrical currents is conducted, e.g., in process control in the electro-winning industry as well as for protection against fault current and revenue metering in electrical power transmission.
Different types of sensing fiber are used in the known sensors. One particular fiber type is spun highly-birefringent fiber, which is produced by rotating the fiber preform of a linear birefringent fiber during the drawing process. Accordingly, the principal axes of the local linear birefringence rotate along the fiber, which introduces elliptical fiber birefringence and makes the near-circular polarization states of the light waves more robust against elasto-optic coupling and hence against perturbing external mechanical stress sources, but still maintains in a wide parameter range sufficient sensitivity to the magnetic field of the current. Thus, usage of spun highly-birefringent fiber as sensing fiber enables simplified packaging compared to a non- or low-birefringent sensing fiber. A spun highly-birefringent fiber is described by the local linear birefringence, for instance expressed by the linear beat length LLB, which is a length section of equivalent unspun highly-birefringent fiber that produces a birefringent phase shift of 2π, and the spin pitch p, which is the length of a spun highly-birefringent fiber section that provides a full revolution of principal axes of the local linear birefringence in the spun highly-birefringent fiber. As trade-off between good robustness and high current sensitivity, these parameters are often chosen so that the spinning ratio x is larger than unity:
  x  =                    2        ⁢                  L          LB                    p        >    1  
Due to the change of the birefringence of the fiber with temperature, the signal as a function of temperature of a fiber-optic current sensor employing spun highly-birefringent fiber can exhibit an oscillatory behavior. This is undesirable as it limits the accuracy of the sensor. It is well known that these instabilities can be sufficiently suppressed by usage of a broadband light source (center wavelength λ, full spectral width at half maximum (FWHM) Δλ) and a sensing fiber of a sufficient length L [4]. In case of a fiber-optic current sensor operated in reflection the sensing fiber length L best meets the following requirement:
            2      ⁢      L        >                  L        EB            ⁢              λ        Δλ              ,
wherein
      L    EB    =                    L        LB                                          1            +                          x              2                                      -        x              ≈          2      ⁢              xL        LB            
for x>1, see e.g. [4, 5]. Typical values are, for instance, LLB=10 mm, p=5 mm, λ=1310 nm, and Δλ=40 nm so that 2L>1.3 m. For a current sensor operated in transmission, the above expression becomes
  L  >            L      EB        ⁢                  λ        Δλ            .      
Obviously, in both cases, this design rule of a certain minimum length of fiber enlarges the sensing coil diameter, if high electrical currents need to be detected, as only a small number of sensing coil windings are allowed in order to stay within the range of detectable magneto-optic phase shifts (e.g. within <±π/2 or <±π), respectively.
It is therefore seen as an object of the invention to provide optical sensors of the above kind, such as fiber-optic current sensors (FOCSs), that reduce adverse effects of fiber stress and simplify the assembly of the sensing fiber coil whilst avoiding reduction in sensor accuracy due to typical characteristics of spun highly-birefringent fiber (oscillatory component in sensor signal at changing temperature) and particularly whilst maintaining a high maximum detectable current for a given minimum length of spun highly-birefringent sensing fiber.
Unrelated to sensors which address the above object, a number of special sensing fiber arrangements for fiber-optic current sensors are known such as:                Designs for fiber-optic current sensors based on a Sagnac-interferometer with counter-propagating waves in the sensor fiber comprising a sensing fiber coil and, in order to reduce vibration sensitivity, a counter-wound fiber coil [6, 7, 8, 18]. As for example stated in Ref. [6], this form of compensation requires the same product of number of counter-wound windings or turns times the area enclosed by the loop or coil as that of the current sensing loop or coil, i.e NsAs=−NcAc (where “s” denotes the sensing coil and “c” denotes the compensation coil. This form of compensation is specific to Sagnac interferometers and not found in reflective interferometers, which are not prone to the Sagnac effect. It should be further noted that to be effective the compensation coil has to be oriented in the same plane as the sensing coil. In most embodiments, the compensation coil is made from a fiber section separated by an optical retarder from the sensing fiber coil.        In [18], also embodiments are disclosed with only partial compensation of the Sagnac effect, i.e. the above mentioned condition on the enclosed area is not fulfilled.        Sensor with two reflective fiber coils with opposite sense of fiber winding [9].        Current sensors comprising a first sensing fiber coil enclosing the primary current conductor and a second coil enclosing secondary electrical windings, wherein the secondary windings are part of a closed-loop feed-back circuit to compensate the magneto-optic phase shift of the first coil [10].        Sensor heads with magnetic screening of the gap between the two ends of the sensor coil [11].        Fiber-optic current sensors to measure the sum or difference of the electrical currents carried in different conductors comprising more than one sensing fiber coil separated by optical retarders and non-sensitive optical fiber [12].        Fiber-optic current sensor with two fiber coils in planes orthogonal to each other, passed by a straight current conductor, with the purpose to eliminate influence of a fiber section with sensitivity to the magnetic field that varies along the length of the section [13].        An assembly for monitoring of a cable consisting of a first and a second fiber-optic leak current sensor, each comprising a first and a second current sensing coil with preferably the same number of fiber windings with opposite sense of rotation. First and second coil of the first sensor are concentrated at a first and a second end of the cable to be monitored. The first coil of the second sensor is concentrated at the first end, while the second coil is distributed over the cable [17].        